An alignment of the Sun and any two planets such that an observer on the outer planet sees the inner planet positioned between himself and the Sun. For example, Venus is at inferior conjunction when it is positioned between Earth and the Sun.

The point at which, as viewed from some planet, an inferior planet lies directly in between the Sun and the planet at which the observation is being made. At this point, the inferior planet is obviously unobservable, unless the line-up is perfect, in which case a transit will be observed.